A eukaryotic cell cycle has a carefully regulated progression of phases: initial gap (G1), DNA synthesis (S), secondary gap (G2), and mitosis (M). G1, S and G2 are known as interphase. In G1, the cell, whose biosynthetic pathways were slowed during mitosis, resumes a high rate of RNA and protein biosynthesis. The S phase begins when DNA synthesis starts and ends when the DNA content of the nucleus has been replicated. The cell then enters G2 where again RNA and protein biosynthesis occur. Following G2, the cell enters M phase that begins with nuclear division and ends with the complete division of the cytoplasm into two daughter cells. This marks the beginning of interphase for the new cells. Non-dividing cells exist at G0, a time following mitosis and before DNA synthesis.
Checkpoint enzymes, such as the serine/threonine protein kinase called checkpoint kinase 1 (CHK-1 or p56CHK-1), are responsible for maintaining the order and fidelity of events in the cell cycle. CHK-1 transduces signals from the DNA damage sensory complex to inhibit activation of Cdc2-cyclin B complex which promotes mitotic entry (Science, 277, 1501-1505 (1997); Science, 277, 1497-1501 (1997)). In eukaryotes, Cdc2 is known as Cdk1 (cyclin-dependent kinase 1). CHK-1 regulates Cdc25, a dual specificity phosphatase that activates Cdc2. Thus, CHK-1 serves as the direct link between the G2 checkpoint and the negative regulation of Cdc2.
Healthy cells have both the G1 and G2 checkpoints and their associated repair processes to ensure viability after treatment of DNA damage (chemotherapy and/or radiation). Cancer cells, however, rely exclusively on the G2 checkpoint and its associated repair processes in order to remain viable and to continue replication. Abrogation of the G2 checkpoint would leave cancer cells with no means to delay progression into mitosis following DNA damage. Inactivation of CHK-1 has been shown to abrogate G2 arrest induced by DNA damage inflicted by either anticancer agents or endogenous DNA damage. In addition, inactivation of CHK-1 results in preferential killing of the resulting DNA damaged, checkpoint defective cells (Cell, 91, 865-867 (1997); Science, 277, 1450-1451 (1997); Nature, 363, 368-371 (1993); Molec. Biol. Cell, 5, 147-160 (1994)). Therefore there is a need for small molecule inhibitors of CHK-1 to preferentially abrogate the G2 checkpoint over G1 and to effectively remove the only checkpoint control found in many types of cancers. When administered during the course of a DNA damaging event, such as chemotherapy employing anti-neoplastic agents, radiation therapy, immunotherapies and antiangiogenic therapies, a CHK-1 inhibitor can sensitize cancer cells thereby triggering damage-mediated apoptosis. Therefore there is a need for a combination therapy involving CHK-1 inhibitor in the course of a DNA damaging event.
Since protein kinases are ubiquitous and interrelated, selective modulation of a single kinase, such as CHK-1, or family of kinases may not result in an effective therapeutic treatment. There is therefore a need for small molecule inhibitors to influence one or more targeted protein kinases whose inhibition, taken as a whole, would produce the desired therapeutic treatment. Although kinase selectivity and its relation to generalized toxicity are important, therapeutic efficacy may rely on the inhibition of more than one protein kinase. Chemical core structures that can be suitably appended to interact selectively and potently with targeted protein kinases represent a valuable tool for drug discovery and scientific research. Therefore there is a need for such a core structure as an inhibitor of one or more protein kinases. Whether administered as a single agent or as co-therapy, the protein kinase inhibitors, such as CHK-1 inhibitors, of the present invention could prove beneficial in the treatment of a number of human diseases, such as cancer.
Certain CHK-1 inhibitors have been proposed for cancer therapy (see Sanchez, Y. et. al. (1997) Science 277: 1497-1501 and Flaggs, G. et al. (1997) Current Biology 7:977-986; U.S. Pat. Nos. 6,413,755, 6,383,744, and 6,211,164; and International Publication Nos. WO 01/16306, WO 01/21771, WO 00/16781, and WO 02/070494).